In recent years, characteristics of organic electronic materials have been remarkably improved. In particular, among low-order conductors such as charge transfer complexes, there are materials having specific properties such as a metal-insulator transition, and application thereof to switching elements for driving an organic EL display panel or to high-density memories is being studied.
Organic bistable materials have attracted attention as materials that can be employed for the aforementioned switching elements. Organic bistable materials are organic materials demonstrating the so-called nonlinear response such that if a voltage is applied to the material, an electric current in a circuit rapidly increases at a voltage above a certain value and a switching phenomenon is observed.
FIG. 19 shows an example of the voltage-current characteristic of an organic bistable material demonstrating the above-described switching behavior.
As shown in FIG. 19, the organic bistable material has two current-voltage characteristics: a high-resistance characteristic 51 (OFF state) and a low-resistance characteristic 52 (ON state), and a nonlinear response characteristic such that if a bias Vb is applied in advance and then the voltage is raised to Vth2 (high transition voltage) or higher, a transition is made from the OFF state to the ON state, and if the voltage becomes equal to or less than Vth1 (low transition voltage), then a transition is made from the ON state to the OFF state and the resistance value changes. In other words, the so-called switching operation can be performed by applying a voltage of Vth2 or higher or Vth1 or lower to the organic bistable material. Here, Vth1, Vth2 can be also applied as pulse voltages.
A variety of organic complexes are known as the organic bistable materials demonstrating such nonlinear response. For example, R. S. Potember et al. fabricated a switching element having two stable resistance values with respect to a voltage by using a Cu-TCNQ (copper-tetracyanoquinodimethane) complex (R. S. Potember et al. Appl. Phys. Lett. 34, (1979) 405).
Further, Kumai et al. used a single crystal of a K-TCNQ (potassium-tetracyanoquinodimethane) complex and observed the switching behavior caused by the nonlinear response (Kumai et al. Kotai Butsuri, 35 (2000) 35).
Furthermore, Ando et al. formed a thin film of a Cu-TCNQ complex by using a vacuum vapor deposition method, clarified switching characteristics thereof and investigated the possibility of application to an organic EL matrix (Ando et al. Preprints of Applied Physics Association Conference, Spring 2000, Vol. 3, 1236).
Further, Yang Yang et al. disclosed that a bistable characteristic for a memory element can be obtained by forming a thin film of a material with a high electric conductivity such as gold, silver, aluminum, copper, nickel, magnesium, indium, calcium, or lithium or introducing it as dispersed fine particles in a material with a low electric conductivity such as aminoimidazole carbonitrile (AIDCN), alumiquinoline, polystyrene, and polymethyl methacrylate (PMMA) and that even if the applied voltage is zero, the preceding ON/OFF state can be stored (PCT Application No. 02/37500).
However, the above-described switching elements using organic charge transfer complexes have the following problems. Thus, because the organic bistable material is a charge transfer complex, it is a material of a two-component system comprising a combination of a molecule with donor properties or a metal element having donor properties and a molecule with acceptor properties such as TCNQ.
For this reason, the composition ratio of the two components has to be strictly controlled when the switching element is fabricated. Thus, in the charge transfer complexes of a two-component system, for example, as shown in FIG. 20, donor molecules and acceptor molecules are stacked as respective columns and form a donor molecule column 61 and an acceptor molecule column 62 and a bistable characteristic is demonstrated because the components of each column carry out a partial charge transition between the molecules (or metal atoms). Therefore, when the composition ratio of the two components is too high or too low, a significant effect is produced on the bistable characteristic of the entire material.
Therefore, in the above-mentioned Cu-TCNQ complex, if the composition ratio of Cu and TCNQ differs, it causes changes in crystallinity and electric characteristics of the materials and a spread in the bistability characteristic. In particular, when a film is formed by a vacuum vapor deposition method or the like, a uniform film is difficult to form over a large surface area because of the difference in vapor pressure between the two components or due to a geometric arrangement employed when separate deposition sources are used for both materials in a co-deposition method. Therefore, in the organic bistable materials in prior arts, there is a problem that it is difficult to mass-product a switching element having a uniform bistable characteristic and product quality. A switching element formed of organic transfer complexes, as shown in FIG. 19, has a problem that transition voltage th2 is high, about 10 V when transition is made from OFF state to the ON state, but the cyclic performance is poor.
Further, the following problem is associated with the switching element described in PCT application No. 02/37500. Though cyclic performance of the element is good, the voltage Vth2 of transition from the OFF state to the ON state shown in FIG. 19 is very low (about 3 V). In particular, the transition voltage is too low in applications to a display drive such as organic EL displays.
The present invention was created to resolve the above-described problems of the conventional technology and it is an object thereof to provide a switching element in which fluctuations of material composition can be inhibited and a uniform bistable characteristic can be obtained, this switching element being suitable for mass production, having a high transition voltage, and demonstrating excellent cyclic performance.